This invention relates to an intermodulation antiinterference device for a superheterodyne receiver. It is a device to prevent the intermodulation interference generated in a high frequency amplifying circuit and a mixing circuit in a receiver. In other words, it detects the intermodulation interference and reduces the intermodulation interference by controlling an attenuating circuit.
When multiple signals having frequencies within the band of the high frequency amplifying circuit enter simultaneously, for example, as in the United States, when VHF band FM broadcasting stations are positioned every 400 KHz, marked intermodulation interference appears when the signals of a neighboring station or next to a neighboring station are large, and the desired signals are disturbed. This intermodulation interference increases when the linearity of the active element of the high frequency amplifying circuit and the mixing circuit is insufficient; when the gain from the antenna terminal to the mixing circuit is high; and when the Q of the selective tuning circuit in the high frequency circuit is low. Therefore, various methods have been devised to eliminate the above-mentioned causative factors by using a low noise, low-gain nonlinear active element or by increasing the Q of the selective tuning circuit.
However, it is impossible in actuality to increase the linearity of the active element of the high frequency amplifying circuit to a sufficient level to oppose a high amplitude interfering wave. As for the active element of the mixing circuit, due to the fact that the mixing circuit relies on the nonlinearity of the active element, the use of an active element with good linearity cancels the primary objective of frequency conversion gain.
Therefore, there are conflicting requirements involved in the prevention of intermodulation interference. When two interfering waves are present near the desired receiving point, and one of the interfering waves is large, the level of the other interfering wave must be lowered considerably. The method for raising the Q in the high frequency circuit is also limited. Another conceivable method is to charge a series resonant circuit between the input terminals of the antenna and the receiver. However, unless a narrow band cyrstal filter that passes only the desired signals is used, there is a limit to the narrowing of the band of a series resonant circuit; and if a crystal filter is used, there is still a problem in that the receiving frequency cannot be varied.
Thus, various measures have their limitations and are unsatisfactory.